1. Field of the Invention
The present invention relates to a porous metal body for use as a battery plate (i.e., a plate used in a battery) or a carrier of any of various substances.
2. Description of the Prior Art
Porous metal bodies each provided with interconnected pores at a porosity of 90% or higher are commercially available which include, for example, CELMET (trade name) produced by Sumitomo Electric Industries, Ltd. This is a porous metal body composed of metallic Ni, being used in various types of filters and plates for secondary alkaline batteries.
The above porous metal bodies have been produced by either the plating process as described in, for example, Japanese Patent Laid-Open No. 174484/1982 or the sintering process as described in, for example, Japanese Patent Publication No. 17554/1963. The plating process comprises coating the skeletal surface of a foamed resin such as urethane foam with carbon powder or the like to thereby render the foamed resin conductive, electrodepositing a metal thereon by the electroplating process and thereafter burning the foamed resin and carbon or the like to thereby obtain a porous metal body. On the other hand, in the sintering process as described in Japanese Patent Publication No. 17554/1963, a porous metal body is produced by impregnating the skeletal surface of a foamed resin such as urethane foam with a slurried metal powder to thereby obtain a slurry-coated composite, drying the composite and heating the dried composite to thereby sinter the metal powder.
Also, a process for producing a porous Al body by casting has been reported (Nikkei Mechanical, 1981/1/5 issue, pages 22 and 23). In this casting-based process, first, a slurry gypsum is cast into a foamed resin such as urethane foam and set to thereby prepare a gypsum mold having a two-dimensional network structure. An Al melt is then cast into the mold, and the gypsum mold is finally removed to thereby obtain a porous Al body.
With respect to major uses of the above porous metal body, recent attention is being drawn to the use as a plate for secondary battery. Actually, the above porous Ni body is being employed in Ni--Cd and Ni-hydrogen secondary batteries. In recent years, a secondary lithium battery is being high-lighted as being suitable for meeting the demand for battery capacity increase. In this secondary lithium battery, the material composing the positive-electrode plate is required to have oxidation and electrolyte resistances because the cell voltage exceeds 3 V. From the viewpoint of the material quality, the porous Ni body cannot be used. Currently, an aluminum foil is being employed as the material for composing the positive-electrode plate, and the use of a porous Al body therefor has been proposed (Japanese Patent Laid-Open No. 28163/1992). In this laid-open specification, lithium or a lithium alloy is used as an active substance of a negative electrode, and it is described that causing the positive-electrode collector to have a porous structure retards the deterioration of discharge capacity by repeating charging and discharging cycles.
Although the materials of most of the conventional porous metal bodies are composed of Ni, the porous body of Ni cannot occasionally be employed in uses requiring lightweight and corrosion and oxidation resistances. Further, in uses as filters, and carriers for battery plates, etc., in which large effective surface areas are required, the porous body of Ni formed according to the plating process cannot be effectively utilized because its typical skeletal sectional form is hollow as shown in FIG. 2(a), so that there is a substantially dead space such as part A being a useless space. In this figure, numeral 10 represents a metal part. With respect to the porous body of Ni prepared according to the sintering process on the other hand, although its sectional form is as shown in FIG. 2(b) and a hollow dead space as shown in FIG. 2(a) is scarce therein, its configuration has a thin skeleton and its surface area (skeletal periphery in FIG. 2(b)) is small, so that from the viewpoint of effectivity its structure also cannot be highly appreciated.
With respect to the production of the porous body of Al, for example, the plating process cannot be applied thereto because Al plating is practically almost unfeasible. Further, it is very difficult in the sintering process to sinter powdery Al having a strong oxide film formed at its surface under atmospheric pressure, so that the process as described in Japanese Patent Publication No. 17554/1963 cannot directly be applied to the production of the porous body of Al. Still further, in the casting process, it is difficult to obtain a porous body having a large number of pores per unit length, i.e., minute pore diameters in view of the productive characteristics of the process.
Although the primary object is attained by the formation of an electrode layer based on a positive-electrode plate of aluminum foil or the like as currently employed in the secondary lithium battery, a plate material is desired which ensures higher reliability, being free from deterioration of output characteristics and capacity irrespective of the repetition of charge and discharge, and which has excellent adherence to electrode substances. That is, generally, the repetition of charge and discharge a number of times causes the positive electrode as a whole to gradually swell to thereby deteriorate the interfacial contact between the core material and the electrode layer with the result that the conductivity of the electrode per se is deteriorated so as to render the attainment of high current density unfeasible and to shorten the duration of charge and discharge cycle. Further, powder comes off from the plate and causes short-circuit, so that there has been a problem in, for example, reliability. These problems have partly been attributed to the occurrence of a reaction causing lithium ions to enter the crystal lattice at the time of charge and discharge reaction with the result that the crystal lattice of the active substance is swollen or shrunk by doping or dedoping with lithium ions so as to bring about the occurrence of defects in the interfaces between the electrode layer and the collector, between the active substance and the plate and between the active substance and the binder resin. Furthermore, a problem of battery reliability is contemplated which is attributed to the deterioration of the active substance layer resulting from the occurrence of local heat caused by the low heat conductivity of each of the electrode materials such as the active substance.
In the proposal (Japanese Patent Laid-Open No. 28163/1992) in which a porous body of Al is used as a plate for suppressing the falling and peeling of the active substance from the positive electrode and thus for improving the charge and discharge cycle characteristics of the nonaqueous-electrolyte-based secondary battery, it is described that the deterioration of discharge capacity by repeating charge and discharge cycle is retarded by the employment of lithium or a lithium alloy as an active substance of a negative electrode and by the employment of a positive-electrode plate having a porous structure. However, the description is limited to average pore diameters, any effective configuration as a porous body is not specified, and there is no clear process for production illustrated.
Any of the prior ark processes do not provide a complete resolution, and the current situation is that any of them can hardly be skated as leading to retention of the cycle life satisfactory for enabling practical use.